Forth

Instructions

Implement an evaluator for a very simple subset of Forth.

Forth is a stack-based programming language. Implement a very basic evaluator for a small subset of Forth.

Your evaluator has to support the following words:

• +, -, *, / (integer arithmetic)
• DUP, DROP, SWAP, OVER (stack manipulation)

Your evaluator also has to support defining new words using the customary syntax: : word-name definition ;.

To keep things simple the only data type you need to support is signed integers of at least 16 bits size.

You should use the following rules for the syntax: a number is a sequence of one or more (ASCII) digits, a word is a sequence of one or more letters, digits, symbols or punctuation that is not a number. (Forth probably uses slightly different rules, but this is close enough.)

Words are case-insensitive.

Implementation

In Forth functions are called words, they operate on a data stack. Each word takes its parameter(s) from the stack and return its result(s) on the stack.

The Forth documentation always contains the state of the data stack before the word is executed and after the word is executed in the following manner ( p1 p2 ... pN --- r1 r2 ... rN ). The values p1 to pN represent the word parameters on the stack before the call (the stack can have additional data further up the stack, not represented here) and the values r1 to rN represent the word return values on the stack after the call. The separator --- distinguish the stack state before and after the word execution.

The words to be implemented are defined as:

• ToS stands for Top of Stack
• The Top of the Stack is represented as the rightmost value

If you want to play with an online forth interpreter or learn more about the words to be implemented try Easy Forth (specifically the Adding Some Numbers, Defining Words, and Stack Manipulation section).